This invention relates to radar jamming systems and more particularly to the use of a primed oscillator in the production of suitable jamming signals for indicating that a xe2x80x9ctargetxe2x80x9d is at a location different from its actual location and is travelling with a velocity different from its actual velocity.
One of the frequently occurring situations necessitating radar jamming is one in which an incoming missile homes on a xe2x80x9ctargetxe2x80x9d via a CW radar signal. This signal, when returned from a xe2x80x9ctargetxe2x80x9d is analyzed as to doppler frequency shift which provides the relative velocity of the xe2x80x9ctargetxe2x80x9d with respect to the missile, and as to modulation which provides the angular bearing of the xe2x80x9ctargetxe2x80x9d. In the past, jammers for this tactical situation have involved the generation of a CW signal by the xe2x80x9ctargetxe2x80x9d in which the frequency of the signal is made to vary in a predetermined fashion to convey false doppler frequency shift information. This is commonly called xe2x80x9cstealing the speed gatexe2x80x9d of the xe2x80x9cenemyxe2x80x9d radar. Additionally, the jamming signal is modulated to convey false angular bearing information. In the prior art jamming devices, it is usually necessary to detect the frequency of the incoming radar signal and to synthetically produce a signal having this frequency. Having generated this signal, the frequency of this signal is slowly varied for the required doppler shift and is modulated to give the false bearing information.
By way of background, there have been basically three approaches to generating the above described CW jamming. The first is a CW amplification approach in which the incoming signal is amplified by a chain of amplifiers and reradiated. In this approach, the false doppler and bearing modulations are applied during amplification and two antennas are required. This approach suffers from two disadvantages. 1) A very high antenna isolation is required to prevent a ring-around type feedback between the antennas. On many installations sufficient isolation is physically impossible to achieve. 2) A separate CW power amplifier tube is required, in addition to the pulse tube which is normally carried for pulse jamming.
The second approach eliminates the antenna isolation problem by using an oscillator to generate the jamming signal. The oscillator is set on frequency by a receiver which briefly samples the received radar signal at periodic intervals. Thus, it will be appreciated that the jamming must be interrupted during these xe2x80x9clook-thru""sxe2x80x9d, so that received radar signals may be sampled. This approach suffers from the disadvantage that the receiver for the jammer is either complex or has a slow response. In addition, a separate CW power tube is still needed.
The third approach makes use of the pulsed power amplifier and simulates CW by a train of pulse bursts. The switching-off between pulses solves the aforementioned ring-around problem, and the time between bursts is available for pulse jamming other threats. However, this approach suffers a severe power/efficiency limitation because the effective power is reduced from the average power by the duty factor. By way of example, typical pulse tubes are limited to a duty factor of about 10%; therefore the effective power is only about 1% of the peak power. It will be appreciated that all of these jammers are carried xe2x80x9con boardxe2x80x9d the xe2x80x9ctargetxe2x80x9d.
In the subject invention, the jammer is also located xe2x80x9con boardxe2x80x9d the xe2x80x9ctargetxe2x80x9d. However, the subject jammer, unlike those of the prior art, includes a single port, keyed, primed oscillator which is primed with the low level signal received from the xe2x80x9cenemyxe2x80x9d radar. The received signal, in one embodiment, is frequency shifted by simple phase shifter to introduce false doppler and then this signal is coupled directly to the tank circuit of the oscillator as the priming signal. The oscillator is rapidly turned on and off (keyed) so that its output signal tracks the frequency of the priming signal as will be explained.
It is important to the understanding of the subject system to distinguish the subject priming system from traditional injection locking systems. In injection locking the injected signal is a relatively high level signal strong enough to lock the oscillator frequency to the frequency of the injected signal. The signal from the xe2x80x9cenemyxe2x80x9d radar normally does not reach injection locking levels at the xe2x80x9ctargetxe2x80x9d and thus injection of a received signal into a remote oscillator circuit will not lock the remote oscillator to the frequency of the received signals.
In priming, however, the injected signal need only be strong enough to force the oscillator to start up in phase with the priming signal at the start of each pulse. In the subject system there is no locking or changing of the resonant frequency of the oscillator. Rather the oscillator is allowed to run at its natural frequency and is rapidly keyed so that the oscillator is turned off before its signal can accumulate a significant phase error with respect to the priming signal. The result is that the output signal from the oscillator tracks the frequency of the priming signal to the extent that the phase difference between the priming signal and the natural frequency signal from the oscillator is small. Since this phase difference can be made very small by rapid keying, the frequency of the output signal from the keyed primed oscillator approximates or tracks that of the priming signal.
Additionally, the oscillator keying circuit may be keyed so that the output signal from the primed oscillator is modulated in such a way as to convey false bearing information.
After these alterations a rapidly pulsed output signal from the jammer is coupled to the same antenna used for receiving the xe2x80x9cenemyxe2x80x9d radar signal, from whence it is transmitted back to the receiving section of the xe2x80x9cenemyxe2x80x9d radar. The xe2x80x9cenemyxe2x80x9d radar cannot distinguish the individual pulses of returned energy due to the high PRF (pulse repetition frequency) of the returned signal and thus the signal from the jammer is detected as a CW signal with false doppler and bearing information thereon. The jammer also has a multiple threat capability in which simultaneously arriving signals from different xe2x80x9cenemyxe2x80x9d radars result in a jammer output at each of the different frequencies of the incoming signals with sufficient jamming power at each frequency.
In summary, in this embodiment the frequency of the priming signal is slowly varied prior to injection into the oscillator to establish a false doppler signal. The oscillator is thus primed with a signal of slowly changing frequency and its output signal reflects the changing input signal to produce a signal with false doppler. To this may be added a second modulation in which the oscillator keying unit may itself be keyed to impart false bearing modulation.
The primed oscillator is therefore a very simple broadband jammer which can follow the incoming frequency by virtue of the rapid keying or chopping. Due to the availability of IMPATT diode oscillators which have outputs exceeding 100 watts, amplification stages are unnecessary. Moreover since the duty cycle can be close to 100%, the effective power out of the oscillator can be very high. Since the subject system utilizes only one antenna there is no isolation problem. It is also an important feature of the primed oscillator that it can be used in its chopped or keyed mode for handling simultaneous multiple incoming signals. This comes about as follows. When two or more CW signals are present, at different frequencies, their resultant is a single signal of varying phase and amplitude. Each time the oscillator is keyed on it is primed by that resultant. Its phase on successive pulses will therefore faithfully follow the phase of the resultant signal, thereby in effect reproducing all the incoming frequencies. The failure of the oscillator output to follow the amplitude variations causes the production of some spurious frequencies. However this is not a problem other than slightly reducing the power available at the desired frequencies.
It is therefore an object of this invention to provide a system for jamming in which a primed oscillator is utilized in a rapidly keyed mode to generate a rapidly pulsed jamming signal having an apparent frequency which duplicates that of the priming signal.
It is another object of this invention to provide a jammer utilizing a rapidly keyed primed oscillator in which the signal from the oscillator is modified in such a manner that the apparent angular bearing and relative speed between the xe2x80x9cenemy radarxe2x80x9d and the xe2x80x9ctargetxe2x80x9d are effective altered, with the modified signal being transmitted by the same antenna that is used for deriving the priming signal.
It is a still further object of this invention to provide a jamming system in which amplifiers or amplification stages are eliminated in favor of a high power primed oscillator which is rapidly keyed to provide an essentially continuous CW signal made up of numerous packets of signals each of which is initially in phase with the incoming radar signal so that the oscillator output signal effectively tracks the frequency of the priming signal.
It is another object of this invention to provide a jamming system utilizing a rapidly keyed primed oscillator having a multi-threat capability in which the simultaneous arrival of two different signals from different xe2x80x9cenemyxe2x80x9d radars results in the production of a signal having frequency components which jam-radars producing the two signals.